This research will investigate behavioral and environmental variables that modify the development and expression of tolerance to the behavioral actions of the narcotic morphine. The experimental methods of behavioral pharmacology will be used to identify ways in which the tolerance produced by acute or chronic morphine pretreatment can be manipulated. The behavioral endpoints assessed will include morphine's effects on ongoing rates and patterns of schedule-controlled operant behavior and morphine's discriminative stimulus properties. Tolerance will be assessed as a decreased effectiveness of the original morphine dose and as a shift to the right in the morphine dose-response curve. The speed or degree of tolerance development will be manipulated by varying the environmental conditions under which behavior is maintained. The proposed research projects have the following aims: l) To determine if the development of tolerance to the rate-decreasing actions of morphine can be accelerated by the opportunity to repeatedly practice the required response following drug administration. 2) To determine if the behavioral consequences of morphine's initial effects can control the development of tolerance to these effects. The effects of differential changes in reinforcement density on the development of morphine tolerance will be assessed. 3) To assess the development of tolerance to the rate-decreasing effects of morphine during establishment of stimulus control by morphine. 4) To identify behavioral conditions under which the development of tolerance to the discriminative stimulus properties of morphine may be assessed and to compare the changes in narcotic discrimination produced by acute or chronic morphine pretreatment with those produced by altering the dose of morphine used to maintain the discrimination. Identification of the behavioral consequences of long-term narcotic administration and of ways in which such consequences may be modified may have implications for the management of narcotic abuse and of narcotic maintenance programs.